1. Field of the Invention
This invention relates to a process and products of this process. More specifically, this invention involves a process for enhancement in the mechanical properties of films prepared from photoconductive polymers, such as poly(N-vinylcarbazole), its homologues and/or its analogues. The films prepared according to this process have enhanced mechanical properties and yet retain substantially the same electrical characteristics of non-oriented films.
2. Description of the Prior Art
The formation and development of images on the imaging surfaces of photoconductive materials by electrostatic means is well known. The best known of the commercial process, more commonly known as xerography, involves forming a latent electrostatic image on the imaging surface of an imaging member by first uniformly electrostatically charging the surface of the imaging layer of said member in the dark and then exposing this electrostatically charged surface to a light and shadow image. The light struck areas of the imaging layer are thus rendered relatively conductive and the electrostatic charge selectively dissipated in these irradiated areas. After the photoconductor is exposed, the latent electrostatic image on this image bearing surface is rendered visible by development with a finely divided colored electroscopic material, known in the art as "toner". This toner will be principally attracted to those areas on the image bearing surface which have a polarity of charge opposite to the charge on the toner particle and thus form a visible powder image.
The developed image can then be read or permanently affixed to the photoconductor where the imaging layer is not to be reused. This latter practice is usually followed with respect to the binder-type photoconductive films (e.g. zinc oxide/insulating resin binder) where the photoconductive imaging layer is also an integral part of the finished copy, U.S. Pat. Nos. 3,121,006 and 3,121,007.
In so called "plain paper" copying systems, the latent image can be developed on the imaging surface of a reusable photoconductor or transferred to another surface, such as a sheet of paper, and thereafter developed. When the latent image is developed on the imaging surface of a reusable photoconductor, it is subsequently transferred to another substrate and then permanently affixed thereto. Any one of a variety of well known techniques can be used to permanently affix the toner image to the copy sheet, including overcoating with transparent films, and solvent or thermal fusion of the toner particles to the supportive substrate.
In the above "plain paper" copying systems, the materials used in the photoconductive insulating layer should be preferably capable of rapid switching from insulating to conductive to insulating state in order to permit cyclic use of the imaging surface. The failure of a material to return in its relatively insulating state prior to the succeeding charging/imaging sequence will result in a decrease in the maximum charge acceptance of the photoconductor. This phenomenon, commonly referred to in the art as "fatigue" has in the past been avoided by the selection of photoconductive materials possessing rapid switching capacity. Typical of the materials suitable for use in such a rapidly cycling imaging system include anthacene, sulfur, selenium, and mixtures thereof (U.S. Pat. No. 2,297,691); selenium being preferred because of its superior photosensitivity.
In addition to anthracene (and its polymers), other organic photoconductive materials, most notably, poly(N-vinylcarbazole) have been the focus of increasing interest in electrophotography, U.S. Pat. No. 3,037,861. Until recently, neither of these polymeric materials have received serious considerations as an alternative to such inorganic photoconductors as selenium, due to fabrication difficulties and/or to a relative lack of speed and photosensitivity. The recent discovery that high loading of 2,4,7-trinitro-9-fluorenone in poly(vinylcarbazole) dramatically improves the photoresponsiveness of these polymers has lead to a resurgence and interest in organic photoconductive materials, U.S. Pat. No. 3,484,237. Unfortunately, films prepared from poly(vinylcarbazoles) have poor mechanical properties, e.g. brittle and inflexible. The addition of high loadings of activators, such as those described in the U.S. Pat. No. 3,484,237 further impairs the mechanical properties of such films.
The orientation of vinylcarbazole polymers is generally known to improve the mechanical properties of these polymers, U.S. Pat. No. 2,215,573. However, where a film of poly(N-vinylcarbazole) is mechanically oriented, the charge transport properties of the resultant film are somewhat impaired. It is believed that during, for example, uniaxial orientation, pendant carbazyl groups are spatially constrained within the composition. It is hypothesized that modification in the steric relationship is carbazyl groups, relative to the plane of the film, is responsible for the observed deterioration in charge carrier transport properties.
While the extent and type of spacial constraint on the pendant groups of photoconductive polymers (which is occasioned during mechanical orientation of films prepared therefrom) has yet to be reported, the technical literature does contain reference to such effects in non-photoconductive polymer systems, see for example M. F. Milagin et al, "Study of Molecular Orientation in Amorphous Polystyrene By Birefringence Methods and By Infrared Spectroscopy", Polymer Science, U.S.S.R. A 12,577-584 (1970). According to the authors of this article, during the uniaxial orientation of polystyrene, the birefringence of the film changes dramatically. This change in birefringence is reportedly attributable to the change in spatial orientation of the pendant phenyl groups relative to the polymer backbone. As the polymer chains are increasingly stretched, the plane of the phenyl groups become increasingly oriented in the direction of the plane of orientation of the polymer film.
It would, thus, appear from the technical literature that the mechanical orientation of films prepared from photoconductive polymers is also impossible without substantial realignment of the pendant photoactive groups relative to the backbone of the polymer chain.
Accordingly, it is the object of this invention to remedy the above as well as related deficiencies in the prior art.
More specifically, it is the principal object of this invention to provide a process whereby the polymer's mechanical properties are enhanced without an accompanying deterioration of electrical properties.
It is another object of this invention to provide a process for enhancing the flexibility of photoconductive polymers.
It is another of the objects of this invention to provide a process for reducing the brittleness of photoconductive polymers.
It is yet another object of this invention to provide a mechanically oriented photoconductive film having good electronic properties.